1. Field of the Invention
This invention relates to a cable assembly comprising a connector terminated to a cable having pairs of twisted conductors, the assembly terminated in such a way as to reduce electromagnetic cross-talk for high speed signal transmission.
2. Description of the Prior Art
There is an increase in demand for cable and connection systems to transmit digital signals at high speeds. As frequency increases, emission of electromagnetic and electrostatic "noise" increases. This is a particular problem for closely positioned conductors, due to electromagnetic interference between the conductors, which is often called "cross-talk". Beyond a certain transmission frequency, cross-talk becomes unacceptably intense (i.e. the signal to noise ratio decreases to an unacceptable level), thus limiting the speed of signal transmission. In cables, one of the way of reducing cross-talk is by twisting pairs of conductors, where one conductor of the pair is for transmitting a positive signal, and the other conductor for transmitting a negative signal of equal intensity and timing as the positive signal. This is called a differential pair due to the nature of the opposed signals in the pair. Due to the twisting about each other, magnetic and electrical field signals emitted from each of the conductors cancel each other out. Similarly, external electromagnetic noise received in the pair cancel each other out. Cross-talk effects are thus reduced in a simple and cost effective manner. Such pairs can thus be placed within a cable and positioned close together whilst nevertheless transmitting high speed electrical signals.
One of the problems however occurs at the connection end, where the conductors are connected to terminal within a connector. Terminals of connectors are often positioned in juxtaposed parallel relationships, thereby causing "untwisting" of the conductor pairs. Cross-talk is thus increased at the connector.
One way of reducing cross-talk effects is shown in European Patent Publication No. 583111 where conductor pairs of a connector are crossed over, thus behaving in a similar manner to that of a twisted cable. Crossing over of contacts in connectors is also shown in U.S. Pat. No. 5,186,647. The latter shows cross-talk reduction in a modular jack, which is a standardized connector widely used in telecommunications and computer data interconnection systems. Standardized modular jacks and corresponding plugs for connection thereto, were initially designed and used for low speed data transmission systems, and are thus not necessarily the most effective connection systems for use with high speed data transmission. Due to their widespread use however, there is a need to improve the data transmission speed capabilities of modular plug and jack connectors whilst respecting the standardized interface requirements.
Another means of reducing cross-talk is by judicious capacitive or inductive coupling between conductors of a connector. Untwisted conductors that are connected to a connector, for example a modular plug, may still pose a problem. Firstly, compensation for cross-talk in a connector is not as good as not being subject to the cross-talk in the first place (for example by maintaining the twist in the wires). Secondly, the length of the exposed and untwisted wire ends that are connected to a connector is often not accurately prepared therefore decreasing the reliability of cross-talk compensation in the connector. For example, cable assemblies may be prepared under field conditions by human operators with varying degrees of skill. The accuracy of conductor end lengths and quality of termination is therefore difficult to guarantee within a tight specification.
It is a continuous requirement to reduce connector assemblies as cost effectively as possible, whilst nevertheless increasing reliability and furthermore increasing the possible speed of signal transmission. It is also desirable to have cable and connector assemblies for high speed signal transmission that interface with or use common industry standards. A good example of the latter is the use of modular plugs and jacks, and cables with pairs of twisted conductors.